Petroleos Mexicanos says the Supremus 1 well is located about 24 miles (39 kilometers) south of U.S. territorial waters, and is more than 4,000 yards (meters) deep.
The company says the area known as the Perdido belt contains prospective reserves that could amount to as much as 13 billion barrels of crude equivalent.
It notes that if proven, that would double Mexico's current proven reserves.
President Felipe Calderon announced the discovery Friday. If brought into production, the well could account for about 10 percent of the company's current daily output, which is about 2.6 million barrels per day.